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ON  THE  CHANGE  OF  REFRANGIBILITY OF  LIGHT.          323
Washed Papers.
87.    In a paper "On the Action of the Eays of the Solar Spectrum on Vegetable Colours," Sir John Herschel mentions a peculiarity which he had observed in paper washed with tine-ture of turmeric, which consists in its being illuminated, when a pure spectrum is thrown on it, to a much greater distance at the violet end than is the case with mere white paper*.    This phenomenon was attributed by Sir John to a peculiarity in its reflecting power, and was considered as a proof of the visibility of the ultra-violet rays.    The colour of the prolongation of the spectrum was yellowish green.    Sir John appears to have been in doubt whether the greenish yellow colour was to be attributed to the mixture of the true colour of the ultra-violet rays with the yellow of the paper due to diffused light, or to the real colour of the ultra-violet rays themselves, which on that supposition would have been incorrectly termed " lavender."
88.    The fact of the change of refrangibility of light having been established, there could be little doubt that the true cause of the extraordinary prolongation of the spectrum on paper washed with tincture of turmeric was very different from what Sir John Herschel had supposed, and that it was'due to a change of re-frangibility in  the  incident light, which was produced  by the medium in a solid state.    Tincture of turmeric has already been mentioned as a medium which possesses in a high degree the property of internal dispersion.    It was the observation of Sir John   Herschel's   already mentioned, which led me to try this medium.    But it is by no means essential that a sensitive substance should be in solution, or in the state of a transparent solid, in order that the change of refrangibility which it produces should admit of being established by direct experiment, although of course the mode of observation must be changed.
89.    A piece of paper was prepared by pouring some tincture of turmeric on it, and allowing it to dry.    In this way the part which was deeply coloured by turmeric was in juxtaposition with the  part which  remained white, which was convenient in contrasting the effects of the two portions.    The sun's light being reflected  horizontally into a darkened room through a vertical
* Philosophical Transactions for 1842, p. 194.
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